The production of S-chloromethyl derivatives of organic dithiophospho acid compounds has suffered economic disadvantages due to the erratic nature of the S-chloromethylation reaction and difficulties in conducting this reaction on a sustained basis to give a high yield of a relatively pure product. Indeed, in order to attain adequate reactivity in a practical production process, the prior art has resorted to the use of a rather expensive chloromethylating agent, namely, bromochloromethane.
Exemplary of the prior art are U.S. Pat. Nos. 3,020,304 (Scherer et al.) and 3,896,219 (Pianka), both of which exclusively employ bromochloromethane as the chloromethylating reagent. The reaction conditions utilized by said patentees are quite similar, with reaction temperatures generally below 100.degree. C. and the usual addition of some inert solvents such as alcohols and ketones. The best results achieved by the two patentees also appear to be quite comparable since Example 2 of U.S. Pat. No. 3,020,304 indicates a yield of about 70 percent of a dithiophosphoric acid derivative using 4 times the stoichiometric proportion of bromochloromethane while U.S. Pat. No. 3,896,219 attained a 75 percent yield of a dithiophosphoric acid derivative using the same proportion of bromochloromethane in the experiment described in the first paragraph of column 7 thereof.
In view of the present state of this art and its virtual dependence upon a single, specialized chloromethylating reagent for practical operations, it would clearly be most desirable to have a satisfactory alternative process employing a more economical and readily available chloromethylating reagent. The present invention is directed to devising a practical process based upon the use of methylene chloride.